1. Field of the Invention
This invention relates to a transpiration cooled blade for a combustion turbine engine and more particularly to a transpiration cooled ceramic blade and the method of its fabrication.
2. Description of the Prior Art
It is well known in the combustion turbine field that as the temperature of the motive fluid for the combustion turbine increases, the efficiency of the engine also increases. However, the temperature of the combustion gases are generally limited because of the inability of the material forming the blades and vanes in the combustion turbine to withstand temperatures greater than approximately 2000.degree. F. To permit combustion gases of a higher temperature, the blades must be cooled to within their allowable operating temperatures. It is now common practice to form the blades and vanes with a high temperature alloy; however, it is also known that blades fabricated from a ceramic material would withstand an even higher temperature and therefore permit a higher temperature for the motive fluid gases with less cooling requirements for the blade, which ultimately yields a much more efficient combustion turbine engine.
There are broadly two distinct methods for combustion turbine blade cooling. The first method is to direct a cooling fluid through internal passages in the blade, permitting the fluid to be discharged into the motive fluid flow path of the turbine, once it has absorbed sufficient heat from the internal structure, through orifices generally in the tip or trailing edge of the blade. A second and more efficient blade cooling method is to deliver a cooling fluid such as air into an internal portion of the blade and permit it to flow through a porous blade surface from both the suction and pressure side of the blade which provides a preliminary cooling effect but primarily envelopes the exterior surface of the blade with a thin film of relatively cool air to prevent impingement thereon of the hot motive gases. This latter method is generally referred to as transpiration cooling.
A transpiration cooled metal blade for a combustion turbine engine is disclosed in U.S. Pat No. 3,810,711 and comprises a porous metal facing preformed to closely fit over the air foil portion of a blade strut and then diffusion bonded thereto. The strut, in addition to being hollow, has orifices formed in the airfoil portion to permit air to escape therethrough and ultimately through the porous facing blade surface.
Although able to withstand a higher temperature, ceramic material is generally brittle. This requires that blades fabricated from ceramic have a substantial cross-sectional area to withstand the centrifugal forces imposed thereon and also have configurations which produce minimal stress concentrations. Methods have been developed for producing solid, monolithic ceramic blades, such as by machining them from solid ceramic billets or by hot pressing them to the desired shape. However, neither of these methods is conducive to producing the internal air flow channels and minute surface orifices needed to distribute the cooling air in the manner required for transpiration cooling. Further, when fabricating a ceramic blade to include air passages and orifices, care must be taken to ensure that the remaining structure has sufficient strength with minimal stress concentrating features to withstand the forces (e.g. both centrifugal force and bending forces) experienced by blades in the combustion turbine engine.